The present invention generally relates to vehicle rearview assemblies and vehicle displays. More particularly, the present invention relates to vehicle rearview assemblies including a transflective (i.e., partially reflective, partially transmissive) mirror element and a high intensity display.
One vehicle accessory that has recently become popular is a back-up assist display which provides a video image to the driver of a scene to the rear of the vehicle where the driver's view may otherwise be obstructed. Sport utility vehicles and trucks have larger areas that are obstructed, and thus particularly benefit from this feature. This feature is a significant safety feature insofar as it helps to eliminate the possibility that someone may back-up over a child or pet playing behind the vehicle or otherwise back-up over an object left behind the vehicle.
In those vehicles that offer this option, the display is typically provided in the instrument panel. More particularly, the display is often provided in the same display that otherwise provides navigation and other information. This enables a single liquid crystal display (LCD) to be utilized in the instrument panel for multiple purposes. Such back-up displays are only activated, and thus viewable, when a driver places the vehicle in reverse. There, it is not practical or economical to provide a large LCD display in the instrument panel that is solely used for the purpose of a back-up display. Another problem associated with placing a back-up assist display in the instrument panel, is that a driver typically looks in the rearview mirror while backing up and not at the instrument panel. In any event, it is difficult to look at both the rearview mirror and the display in the instrument panel at the same time.
Accordingly, it has been proposed to place the back-up assist display in the rearview mirror assembly such that the driver may readily view the images provided by the mirror element and the back-up assist display at the same time. The provision of a video display in a rearview assembly for providing images of a scene to the rear of a vehicle has been disclosed in commonly assigned U.S. Pat. No. 6,550,949 and U.S. Patent Application Publication No. 2003/0103141 A1. Although it has been proposed to incorporate such video displays in a rearview mirror assembly, such proposals have not been adopted by the auto industry for several reasons.
In such proposals, it has been proposed to remove the reflective layer of the mirror element in the region in front of the LCD video display in order to enable an image of sufficient brightness to be displayed to a driver during high ambient conditions such as daytime. However, this leaves a large area of the mirror surface without a reflective layer such that when the mirror element is being used in the conventional sense, it appears much smaller to the driver. Thus, another proposal is to place the display behind what is known as a “transflective” layer provided on a mirror element. A “transflective” layer is a mirror layer that is partially reflective and partially transmissive generally across the entire visible spectrum so as to enable the full color display to be fully visible by the driver. While this latter proposal is very advantageous in that the mirror appears as a normal mirror when the display is not utilized, a difficulty arises insofar as commercially available automotive grade LCD displays are not bright enough to provide a sufficiently bright image to the driver of the vehicle. This is because a significant percentage of the light emitted from the display is attenuated by the transflective layer. A further problem results in that the transflective layer reflects a high percentage of the ambient light striking the mirror from the rear of the vehicle, and such reflected ambient light tends to wash out the image from the LCD display. This results in the display not having sufficient contrast ratio for the driver to pick out possible obstructions to the rear of the vehicle.
To be utilized as a video display in a rearview mirror assembly, an LCD display must be “automotive grade” and generally should provide a high contrast image of greater than 400 candelas per square meter (cd/m2). It should be noted that navigational LCD displays generally have light outputs of 500 cd/m2. Again, however, given that an interior rearview mirror assembly is typically required to have a reflectance of at least 60 percent, a transflective mirror would normally have a transmission of 20 percent, meaning that the placement a conventional LCD display having an output of 500 cd/m2 would only produce a light output of 100 cd/m2 at most when placed behind the transflective mirror element. This is unacceptable given the additional problem of the decreased contrast ratio resulting from providing a mirrored surfaced in front of the LCD display.
It should further be noted that not all LCD displays are “automotive grade.” To be “automotive grade” means that the LCD display must be designed to operate in an automotive environment. Such displays are ruggedized and have a high tolerance for shock and vibration, wide operating and storage temperature ranges, high radiated emissions susceptibility, and high brightness. Typical specifications for “automotive grade” displays are:                a. Operating Temperature Range −35° C. to +85° C.        b. Storage Temperature Range −40° C. to 95° C.        